The present invention relates to a cryostat, in particular a helium cryostat with an additional nitrogen tank and a vacuum section, comprising a pressure-relief cover arranged at the outside of the tank wall of the cryostat for closing an opening therein.
Cryostats of this type are employed, for example, for housing the superconductive magnet coil of an NMR spectrometer. As is generally known, the cryotank containing the cryogen fluid of a cryostat is enclosed by a vacuum jacket. The vacuum chamber is evacuated for thermal insulation of the cryotank, and the cryotank is additionally enclosed by a superinsulating film. For evacuation of the vacuum chamber, the latter is provided with a vacuum valve through which the vacuum chamber communicates with a vacuum pump and which can be closed after evacuation of the chamber. In addition, the vacuum chamber is equipped with a pressure-relief cover protecting the vacuum chamber from inadmissibly high pressures. In the evacuated condition of the vacuum chamber, the pressure-relief cover is urged against the tank opening of the cryostat by the atmospheric pressure, or the environmental pressure so as to close the opening via a sealing ring, normally an O ring. If during operation of the cryostat the cryotank gets untight, cryogen liquid will get into contact with the warm vacuum jacket and vaporize. The rise in volume of the cryogen fluid resulting from its transition from the liquid into the gaseous phase, leads to a pressure rise in the vacuum chamber. At the point where this pressure exceeds the atmospheric or environmental pressure, the pressure-relief cover is lifted off and the gas is permitted to escape through the tank opening of the cryostat into the open air so as to prevent the vacuum tank from bursting and to avoid the risks resulting therefrom. If, however, a small cross-section is released only when the cover is lifted off, there is a risk that fragments of the superinsulating film which may have got detached may block the outlet opening and prevent further gas from escaping.
If the pressure-relief cover is held on the tank opening of the cryostat merely by the action of the atmospheric pressure, then there is always the risk that the pressure-relief cover may be lifted off as soon as an overpressure condition occurs in the vacuum chamber, and may be completely detached due to its own weight so that a permanent connection may be created between the vacuum chamber and the surrounding air. This normally occurs when an overpressure builds up in the vacuum chamber during heating-up of the empty cryotank--which is normally effected by flooding the vacuum chamber with dry nitrogen gas. Then cold gas, which previously had frozen in the vacuum section of the cryostat, at the surface of the inner liquid helium tank, may start to melt and to vaporize. However, the resulting development of gas is not as violent as in the case of a leakage in the cryotank. If frozen nitrogen gas starts melting and vaporizing the pressure-relief cover should open only in order to release the excess pressure whereafter the opening of the tank should be closed again to protect the tank from humidity, air, dirt, etc., as any humidity would immediately condense on the cold superinsulating film and would thereby affect the latter's effectiveness.
There have been known arrangements where the cryostat tank is provided with a pressure-relief valve of small cross-section and an additional bursting disk of large cross-section. The pressure-relief valve serves the function to release minor overpressure conditions by opening instantaneously. If, however, a higher overpressure prevails in the tank, which can no longer be released by the pressure-relief valve, then the bursting disk will break thereby creating a relief opening of large cross-section. This safety means is, however, rather complicated as the tank has to be equipped with two devices, and is in addition cost-intensive because the bursting disk, being a non-reversible safety device, has to be replaced every time the device is activated.
The bursting disk may also be designed in such a way that it will be flung off the container in the presence of excessive pressure. In order not to endanger the operating personnel in such an event, bursting disks of this type are normally arranged at the tank bottom. However, this arrangement is connected with the disadvantage that the bursting disk will drop off the tank already in the presence of minor overpressures in the tank. In order to remedy this disadvantage, one normally fixes the disk on the tank, for example by means of a glue or an adhesive tape. Such an adhesive bond has, however, an influence on the point in time at which the disk is to be lifted off in the presence of a given pressure inside the tank, and this condition may give rise to accidents.